DESCRIPTION (from applicant's abstract): The annual change in day length is the major environmental factor regulating the timing of reproduction in many seasonally breeding mammals. Both the circadian clock located in the hypothalamic suprachiasmatic nucleus and the pineal melatonin rhythm play a central role in relaying information about length of the day to the neuroendocrine-gonadal axis. Despite the fact that a variety of age-related changes in the circadian clock system have now been well documented in mammals, including changes in the response to the entraining effects of the light dark cycle and dampening of the pineal melatonin rhythm, very little is known about how age-related changes in circadian function may impact on the response of seasonally breeding animals to changes in day length. In the proposed studies, golden and Siberian hamsters will be utilized to test various hypotheses regarding 1) the effects of age on the photoperiodic control of neuroendocrine-gonadal activity and the circadian clock system, 2) how aging alters the interactions between the various components of the photoperiodic time measuring system and 3) how day length influences the effects of age on circadian rhythmicity. For all studies the rhythm of locomotor activity will be used as a marker for the state of the circadian clock system. Serum LH, FSH and testosterone levels, as well as testicular size, will be monitored to assay the state of the reproductive system, and in some studies serum melatonin levels will be determined. Various studies are proposed to determine how aging may effect 1) the critical day length for the photoperiodic response and the entrainment to different light-dark cycles, 2) the pattern of melatonin secretion, 3) the response of the neuroendocrine-gonadal axis to melatonin, 4) the sensitivity of the neuroendocrine-gonadal axis to light intensity and 5) feedback relationships in the circadian clock system and in the neuroendocrine-gonadal axis. Other studies will determine if age-related changes in the photoperiodic response can be blocked by stimuli that alter the circadian temporal organization. Taken together, the proposed studies are expected to yield new information about the physiological mechanisms that underlie age related changes in the photoperiodic response. Age related changes in the way hamsters respond to changes in the length of the day offer new avenues for elucidating the physiological and cellular mechanisms that underlie the generation of circadian rhythms as well as the photic control of the hypothalamic-pituitary-gonadal axis. In view of recent demonstrations that the pattern of human endocrine rhythms is also influenced by the length of the day, the results of the proposed studies are expected to increase our understanding of how age-related changes in the human neuroendocrine system may be affected by the seasonal change in day length; such information may also lead to new treatments of mood disorders that have been associated with seasonal rhythms.